A significant environmental problem for many hydroelectric facilities is the water quality of turbine discharges. The primary water quality problem is often low dissolved oxygen (DO) levels which occur seasonally. During the summer months, thermal stratification of the reservoir produces a surface layer of less dense, warm water with relatively high dissolved oxygen and a lower, relatively cold oxygen-depleted layer resulting from the decay of organic material on the reservoir bottom. Because hydraulic turbines draw their discharges from the lower layer, this can create water quality problems downstream of the facility.
Relicensing and rehabilitation of a hydroelectric facility offer an opportunity to address environmental and industrial development concerns over dissolved oxygen levels and other water quality regulations which affect hydropower releases. Rehabilitating an existing hydroelectric facility may, as explained in co-pending U.S. patent application Ser. No. 08/733,366 filed Oct. 17, 1996 to the same assignee, include replacement of the runner. Replacing an existing runner with a new runner having integral passages, and providing air through existing structures or new structures of the installation to the integral passages, enhances dissolved oxygen levels in the water without material losses in efficiency or substantial increases in cost of rehabilitation.
Another known way to improve water quality in turbine installations having shrouded runners is to supply air to the annular chambers or spaces formed between the stationary casing and the runner crown and band. In co-pending U.S. patent application Ser. No. 08/805,286 filed Feb. 25, 1997 to the same assignee, such chambers are specifically configured to promote the continuous mixing of a cloud of small gas bubbles and to maximize the residency time of the bubbles.
It is also known to admit air through the foundation to the draft tube. Providing air admission to the draft tube has heretofore been done, however, primarily to quiet pressure pulsations and cavitation problems caused by part load or overload vortices within the draft tube. In some designs, this was accomplished by providing an annular air/water mixture chamber around the draft tube and a finite number of discrete outlets connecting the chamber to the draft tube. Although this compensated somewhat for shifting local pressure pulsations in the tube, no attempt was made to ensure an even distribution of the gas into the tube, as is desirable to increase dissolved oxygen levels in the turbine release.
Accordingly, it appears desirable to develop new gas distribution arrangements in the draft tube to improve the water quality of hydropower releases, and thereby as an example increase the survivability of fish downstream of such turbine installations.